Integrins are cell surface glycoprotein receptors that bind extracellular matrix proteins and mediate cell-cell and cell-extracellular matrix interactions (generally referred to as cell adhesion events). These receptors are composed of noncovalently associated alpha (α) and beta (β) chains that combine to give a variety of heterodimeric proteins with distinct cellular and adhesive specificities. Certain integrins have been implicated in the regulation of a variety of cellular processes including cellular adhesion, migration, invasion, differentiation, proliferation, apoptosis, and gene expression.
The αvβ6 receptor is one member of a family of integrins that are expressed as cell surface heterodimeric proteins. While the αv subunit can form a heterodimer with a variety of β subunits (β1s β3, β5, β6 and β8), the β6 subunit can only be expressed as a heterodimer with the αv subunit. The extracellular and cytoplasmic domains of the β6 subunit mediate different cellular activities: the extracellular and transmembrane domains have been shown to mediate TGF-β activation and adhesion; whereas the cytoplasmic domain of the β6 subunit contains a unique 11-amino acid sequence that is important in mediating αvβ6 regulated cell proliferation, MMP production, migration, and promotes survival.
αvβ6 can bind to several ligands including fibronectin, tenascin, and the latency associated peptide-1 and -3 (LAP1 and LAP3) (the N-terminal 278 amino acids of the latent precursor form of TGF-β1). The TGF-β cytokine is synthesized as a latent complex in which the N-terminal LAP is non-covalently associated with the mature active C-terminal TGF-β cytokine. The latent TGF-β complex cannot bind to its cognate receptor and thus is not biologically active until converted to an active form. αvβ6 binds LAP1 and LAP3 through interaction with an arginine-glycine-aspartate (“RGD”) motif and this binding of αvβ6 to LAP1 or LAP3 leads to activation of the latent precursor form of TGF-β1 and TGF-β3 as a result of a conformational change in the latent complex allowing TGF-β to bind to its receptor. Thus, upregulated expression of αvβ6 can lead to local activation of TGF-β, which in turn can activate a cascade of downstream events.
The TGF-β cytokine is a pleiotropic growth factor that regulates cell proliferation, differentiation, and immune responses. TGF-β also plays a role in cancer. TGF-β is recognized to have tumor suppressor and growth inhibitory activity, yet many tumors evolve a resistance to growth suppressive activities of TGF-β. In established tumors, TGF-β expression and activity has been implicated in promoting tumor survival, progression, and metastases. This is thought to be mediated by both autocrine and paracrine effects in the local tumor-stromal environment, including the effects of TGF-β on immune surveillance, angiogenesis, and increased tumor interstitial pressure. Several studies have shown the antitumor and anti-metastatic effects of inhibiting TGF-β.
The αvβ6 integrin has multiple regulatory functions in tumor cell biology. The expression of αvβ6 is restricted to epithelial cells where it is expressed at relatively low levels in healthy tissue and significantly upregulated during development, injury, and wound healing. αvβ6 is upregulated on cancers of epithelial origin, including colon cancer, squamous cell cancer, ovarian cancer, and breast cancer.
Murine αvβ6 antibodies have been shown to be able to effectively inhibit tumor growth in a human tumor xenograft model. Furthermore, several murine αvβ6 antibodies have been shown to be effective in preventing fibrosis of the kidney and lung. However, murine antibodies are unsuitable for treatment of human subjects because murine antibodies can evoke anti-mouse immunoglobulin antibody production in the human subject's body. Reduction of the immunogenicity of therapeutic antibodies is desirable because induction of an immune response can cause a spectrum of adverse effects in a subject, ranging from accelerated elimination of the therapeutic antibody with consequent loss of efficacy to fatal anaphylaxis at the most extreme.
Accordingly, there is a need to develop αvβ6 antibodies that are less antigenic in humans and that are useful in the treatment of diseases involved in the αvβ6 pathway, such as acute tissue injury, fibrosis and cancers.